(i) Field of the Invention
The present invention relates to a method for preventing the decomposition of ammonia, and a device for injecting ammonia in which the decomposition of ammonia can be prevented. More specifically, it relates to a method and a device for preventing the decomposition of ammonia before ammonia has reached a denitration apparatus, when an exhaust gas is denitrated at high temperatures in the denitration apparatus using ammonia as a reducing agent.
(ii) Description of the Related Art
In a denitration apparatus utilizing selective contact reduction which is installed to treat an exhaust gas coming from a gas turbine or a diesel engine, a steel pipe made of soft iron has been used as a device for injecting a reducing gas, such as ammonia, for a mid-temperature denitration process in which the temperature of the exhaust gas is 400.degree. C. or less. Also, in a high-temperature denitration process in which the temperature of the exhaust gas is 480.degree. C. or more, a device for injecting ammonia as the reducing agent, such as an injection mother pipe, an injection nozzle or the like disposed in an exhaust gas duct, is heated by the exhaust gas, so that ammonia is heated up to high temperatures. In such a high-temperature denitration process, a steel pipe made of a chrome-molybdenum steel or a stainless steel is used in consideration of heat resistance, but ammonia is brought into contact with the iron component of the injection pipe and therefore ammonia tends to become oxidized and decomposed.
As a result of this decomposition of the ammonia gas, the actual amount of ammonia fed to the inlet to the denitration catalyst becomes reduced, and thus an excessive amount of ammonia beyond the amount required solely for denitration has to be supplied. In addition, the decomposition of the ammonia gas leads to the generation of nitrogen oxides (hereinafter abbreviated to "NO.sub.x ") and also gives rise to a reduced denitration efficiency.
As described above, in the case where the ammonia gas is injected into a high-temperature atmosphere at 480.degree. C. more, the ammonia gas comes in contact with the inner surface of an injection pipe made of a chrome-molybdenum steel or a stainless steel or with a metal component of a nozzle structure and decomposes. Due to this decomposition of ammonia, NO.sub.x is produced. Therefore, when such a process is applied to an apparatus for a denitration reaction, the denitration efficiency deteriorates. In order to avoid this disadvantage, it is necessary to cover metal-gas contact portions exposed to the high-temperature atmosphere with a material which does not cause a reaction with the ammonia gas even in a high-temperature range. In addition, the apparatus for the denitration reaction must have a structure which has mechanical strength and can withstand thermal shocks at the high temperature.